Cell phones are designed to operate within certain voltage parameters. Exceeding the specified maximum positive or negative voltage will damage the internal circuitry of the phone. When voltages are applied to integrated circuits that are beyond the specified maximum, Electrical Over Stress (EOS) damage occurs. EOS damage causes the integrated circuit to fail and disables the phone.
The following three examples describe possible situations that could cause EOS damage. First, external batteries and chargers provide power for operation. If the external battery or charger is defective or designed improperly, it can supply voltages beyond the specified maximum voltage. Second, during phone production, external test jigs stimulate the phone with different voltages while checking operation. If the test jig is improperly designed, or out of specification, the specified maximum voltage can be exceeded. Third, end customers may accidentally apply voltages exceeding specifications by using unapproved third party accessories. Additional situations exist that can also cause detectable EOS damage.
Phones and devices exposed to the described EOS inducing situations can fail. Phone manufacturers currently return failed phones and devices to the device manufacturer. Failure Mode Analysis (FMA) is performed on the devices to determine the defect. One type of failure mode is EOS, as mentioned above, and is determined by the failure signature observed on the integrated circuit. Several situations could Cause EOS damage. Manufacturers try to identify the probable cause of the EOS damage by applying different over voltage conditions to several devices and performing FMA analysis on each one. Once an experimental device shows similar characteristics to the customer-failed device, the manufacturer can theorize the cause.
However, considerable FMA work is required to detect EOS damage induced in the described manners. Non-working phones and devices are often returned with no explanation of how the failure occurred. Final end customers either do not know or are unwilling to disclose how the EOS damage is induced. The OEM intermediate customer assumes that the phone was never used out of compliance and requires the manufacturer to explain how their devices could not withstand “normal” operating environments. Only circumstantial evidence of the EOS failure indicates that voltages exceeding the maximum positive or negative specified voltage have been applied to the devices. There is never any hard proof to indicate such an event, and therefore, the customer is never completely satisfied with the manufacturer's response to the submitted failure.
In addition, it is very time consuming to perform the experiment of damaging a device in a predicted manner and then performing FMA analysis to determine if it matches the original customer-failed device. Each experiment cycle can add 7 business days. It is not uncommon to have several experimental cycles, so 14-20 days can easily be added to the evaluation interval. EOS damage is usually considered critical and could shut down a factory assembly line resulting in lost revenue for both companies. Solving the problem as quickly as possible saves both the manufacturer and the retailer money.
Lastly, performing FMA is expensive. Considerable cost can be saved if only one FMA cycle is needed to determine the root cause of the EOS failure. If multiple FMA cycles are needed on new customer-failed devices and experimental devices, the cost can be significant. Solving the EOS failure in one FMA cycle provides considerable cost savings.
The current methods do not identify the exact cause of the failure. Instead, they provide circumstantial evidence that matches the type of damage seen on the customer-failed device. The evidence is not conclusive and does not decisively identify the root cause of the failure. If a customer has a clearly identified root cause, they can search their process or use-cases to determine how the condition causing the failure occurred.